Optical Coherence Tomography (OCT) is a technology that allows for non-invasive, cross-sectional optical imaging in biological media with high spatial resolution and high sensitivity. OCT is an extension of low-coherence or white-light interferometry, in which a low temporal coherence light source is utilized to obtain precise localization of reflections internal to a probed structure along an optic axis. In OCT, this technique is extended to enable scanning of the probe beam in the direction perpendicular to the optic axis, building up a two-dimensional reflectivity data-set, used to create a cross-sectional gray-scale or false-color image of internal tissue backscatter.
OCT has been applied to imaging of biological tissues in vitro and in vivo, although the primary applications of OCT developed to date have been for high resolution imaging of transparent tissues such as the eye. In an OCT image, the detectable intensities of the light reflected from layers of most thick scattering tissues range from 10.sup.-5 to 10.sup.-10 th part of the incident power. For most of these studies, compact and inexpensive super luminescent diode sources (SLDs) have been used as interferometer illumination sources. These commercially available SLDs provide ranging resolutions of 15 to 20 .mu.m (full width at half maximum, i.e., FWHM) in free space. In order to probe ultra-structural details in tissues with fine detail, higher longitudinal resolution is desirable. This is particularly important in the case of non-invasive medical diagnostics, since it is useful to obtain high depth resolution imaging using compact and inexpensive sources which could be easily integrated with endoscopes and catheters.
Epithelial cancers of the breast, lung, and GI tract comprise over 50 percent of all cancers encountered in internal medicine. Many epithelial cancers are preceded by premalignant changes, such as dysplasia or adenoma. Most early GI cancers originate in the superficial layers (i.e., mucosa and submucosa) of the gastrointestinal tract. Because the depth range of OCT imaging is 2 to 3 mm, OCT is sufficient to penetrate superficial tissue layers lining all internal and external free surfaces of the body, including vascular, respiratory, and GI systems, as well as the skin. If axial resolution of OCT images can be optimized to provide cellular resolution (i.e., in the order of 5 .mu.m), OCT could be used in accurate GI cancer staging and high fidelity diagnosis of precancerous diseases such as Barrette's esophagus and chronic ulcerative colitis.
Because the axial resolution is a function of the coherence length of the low coherence source, typically on the order of 15 .mu.m, one known attempt to gain high depth resolution has utilized an ultra-short pulse laser as an alternative source of low coherence length illumination. Ranging resolution of 3.7 .mu.m FWHM has been reported using femtosecond Kerr-lens modelocked TI:Al.sub.2 O.sub.3 laser illumination. A disadvantage with these femtosecond sources is that they are very complicated and expensive, and their medical usage still remains difficult. Accordingly, a need exists for a system that can achieve the high depth resolution imaging utilizing the compact and inexpensive SLDs which can be easily integrated in endoscopes and catheters.
Another problem in known OCT systems, is the formation of unwanted speckle noise in the final gray-scale or false-color image. This speckle noise is caused by the existence of closely spaced reflecting or backscattering sites (located within a coherence length of the SLD to each other) within the sample. Speckle is caused by destructive or constructive interference between the waves backscattered from closely spaced reflecting sites. Because prior art OCT systems have detected only the envelope (i.e., the magnitude data) of the interferometric signal, these systems are unable to resolve the interference between the closely spaced reflectance sites, often producing inaccurate positioning of reflections in addition to spurious reflections in the final image. Accordingly a need also exists for a system that can resolve the closely spaced reflectance sites in the sample, so as to substantially eliminate speckle noise in the final image.